The present invention relates to radio navigation systems generally and, more particularly, to such systems utilizing an element antenna in which said elements are arranged in a circle and adapted for receiving direction finding or active omnidirectional operation.
A radio navigation system of the general type is known, for example, from German Pat. No. 1,123,000. The radio navigation system described therein can be an omnidirectional range or a direction finder. A direction finder is basically a receiving system which determines the angle of arrival of a received signal. An omnidirectional range radiates a signal from which the bearing to that facility can be determined by airborne apparatus.
Of the various prior art approaches for omnidirectional range implementation, VOR and TACAN are two which have been introduced throughout the world and are of great importance in modern air navigation. The signals radiated from those omnidirectional range facilities must meet requirements set forth in rigid specifications.
The principle of an omnidirectional range involves ground station transmission of a direction-dependent signal which is evaluated as azimuth information with the aid of a suitable airborne receiving apparatus. In VOR (VHF omnidirectional range), which will be explained representatively for the general class of omnidirectional ranges, the direction-dependent signal is a 30 Hz wave whose phase position relative to a reference wave corresponds to the azimuth. By rotating a VHF figure-8 directional pattern (e.g. of a dipole) at 30 rps (transmitted frequency range 108 MHz to 118 MHz) a 30 Hz amplitude modulation (AM) is effectively produced in the field, the direction-dependent signal thus being radiated. For the azimuth evaluation in the airborne receiver, the ground station transmits an additional 30 Hz reference signal. The latter is contained as frequency modulation (FM) in a 9,960 Hz subcarrier wave with a frequency deviation of .+-.480 Hz, with which the aforementioned VHF carrier is amplitude-modulated. In this manner, the direction-dependent signal and the reference wave, which are of the same frequency, are well isolated from each other. The airborne receiver evaluates the phase difference between these two 30 Hz waves; this difference being the azimuth or bearing information. Phase equality between the direction-dependent and reference signals is pre-set in the direction of magnetic north, typically. The carrier is also amplitude-modulated with speech (300 Hz to 3,000 Hz) ground station and identification (1,020 Hz).
Depending on the surroundings at the site of the omni-range station, site errors occur during evaluation. These errors are caused by reflection of the radiated signal from obstacles in the propagation path and may produce unacceptably large errors. In severe cases, information containing the azimuth of the reflector in addition to the direct azimuth information appears at the receiving station. Vector addition produces an error whose magnitude is dependent on the relative amount of reflected radiation, on the radio-frequency phase difference between direct and reflected signals, and on the difference between receiver and reflector azimuths.
Because of these possible errors, a VOR must always be located in a place where the site error is negligible. Such a location cannot always be found, however. One further development of VOR is the so-called Doppler VOR, which affords great reduction of site error. Doppler VOR is described in the above referred-to patent specification and is compatible with the VOR. The Doppler VOR can be located even on terrain with many obstructions.
By reciprocity, it will be realized that in direction finders, too, the measurement result may be invalidated by errors caused by multipath propagation if the facility is located on terrain with many obstructions. By application of the Doppler principle, much in the same way as in the Doppler VOR, these errors are nearly eliminated. Doppler direction finders are also described in the aforementioned reference.
Notwithstanding their performance advantages, both the Doppler VOR and the Doppler direction finder are wide-base systems and, therefore, occupy much space. In addition, those radio navigation systems are very expensive to construct and operate. The text "Funksysteme fur Ortung und Navigation", Verlag Berliner Union GmbH, Stuttgart, FRG, 1973 (pages 131 to 139) and the text "Electronic Avigation Engineering" by Peter C. Sandretto, published by International Telephone and Telegraph Corporation, 1958, are useful references for VOR background.